The increases that have taken place in the capacity of various types of optical discs have been achieved by reducing the size of the recorded marks by which information is recorded on the tracks on the disc, and by reducing the focused spot size on the focal plane by using laser light of shorter wavelengths for recording or reproducing, and objective lenses of larger numerical aperture.
For example, the CD (compact disc), with a disc substrate functioning as a light transmitting layer substantially 1.2 mm thick (including a transparent protective layer disposed on the information recording layer and a spacer layer; also referred to as a transparent substrate), a laser beam wavelength of substantially 780 nm, and an objective lens with a 0.45 numerical aperture (NA), had a capacity of 650 MB.
The DVD (digital versatile disc), with a light transmitting layer substantially 0.6 mm thick, a laser beam wavelength of substantially 650 nm, and a 0.6 NA, had a capacity of 4.7 GB.
The higher density BD (Blu-ray disc), using an optical disc with a light transmitting layer substantially 0.1 mm thick, a laser beam wavelength of substantially 405 nm, and a 0.85 NA, has achieved the large capacity of 25 GB per layer.
High density recording systems that use super-resolution optical discs have recently been under study in the optical recording field; in a super-resolution optical disc, a super-resolution mask layer having a nonlinear light absorbance characteristic or nonlinear light transmittance characteristic in which the refractive index changes with the light intensity is formed. These systems can reproduce marks smaller than the λ/(4NA) diffraction limit determined by the wavelength λ of the light and the numerical aperture NA of the focusing lens, which is an optical element of the optical disc device, by causing a change in refractive index in a localized area of high light intensity or high temperature in the focused spot on the optical disc. The localized area in which, for example, the change in refractive index occurs will be referred to simply as the aperture below (see, for example non-patent document 1).
Non-patent documents 1 and 2 concern a typical super-resolution optical disc of the Super-RENS (Super REsolution Nearfield Structure) type. Super-resolution optical discs in which the recorded marks of information are formed by a material having a nonlinear light absorbance characteristic or nonlinear light transmittance characteristic such that the refractive index changes with the light intensity have also been proposed (see, for example, non-patent document 3). The term super-resolution optical disc will be used below to refer to all of these.